1. Field of the Invention
The present invention relates to a sheet post-processing apparatus that applies a post process to a sheet, and particularly, relates to a sheet post-processing apparatus that is used with an image forming apparatus, such as a copying machine or a laser beam printer, and a control method therefor.
2. Description of the Related Art
A sheet post-processing apparatus (a finisher) is arranged at the downstream position of an image forming apparatus (a copying machine etc.) in a sheet conveyance direction, and applies post-processes, such as a staple process and a punching process, to a conveyed sheet bundle.
Such sheet post-processing apparatuses sequentially stacks sheets that are received from an image forming apparatus onto a processing tray that is arranged at the upstream position of a stacking tray. In such an apparatus, stacked sheet bundle is aligned by adjusting both sides of sheets in a width direction perpendicular to the sheet conveyance direction by alignment members. After all the sheets that comprise one booklet are stacked on the processing tray, a post process, such as a staple process, is performed on the processing tray. Then, the sheet bundle to which the post process was performed is ejected from the processing tray to the stacking tray (see Japanese Laid-Open Patent Publication (Kokai) No. H10-181988 (JP H10-181988A)).
The sheet post-processing apparatus disclosed in this publication is provided with a skew feeding roller for giving a sheet the moment toward one of the alignment members in order to improve the adjustment of the sheets on the processing tray.
Incidentally, the sheet post-processing apparatus disclosed in JP H10-181988A is assuming that all the sheets included in the sheet bundle that is the target of the alignment process on the processing tray have the same width.
However, actual sizes (actual measurement) of sheets of some manufacturers or brands are often different from nominal sizes. In addition, there may be lot-to-lot unevenness or sheet-to-sheet unevenness in sheets of the same brand. It is known that the degree of shrinkage of a sheet after fixing by an image forming apparatus varies depending on the material type of the sheet.
Under the condition where sheets of different brands or material types are mixed in one booklet, when an alignment process is performed with reference to the sheet with the maximum width, it becomes difficult to align sheets with the smaller width than the maximum width. As a result, there is a possibility that a booklet is generated in the state where some sheets are in unfixed positions between the alignment members and are not parallel.
On the other hand, when the alignment process is performed with reference to the sheet with the minimum width, there is a possibility that an edge of the sheet with the larger width than the minimum width is wrinkled or torn.
The sheet post-processing apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H7-172672 (JP H7-172672A) is able to stack sheets on a processing tray by giving a sheet the moment toward one of the alignment members as a reference position by a skew feeding roller of the processing tray even if the sheet width is uneven.
However, it is necessary to change the reference position depending on contents of post process or a position at which the post process is performed. For example, when one side of sheet bundle is bound at two positions, it is preferable that the “bottom” of the booklet is the reference position in consideration of keeping the booklet in a bookshelf.
However, the top and bottom of the sheet bundle stacked on the processing tray are changed according to which of the right-side and left-side opens is selected by a user in the case of the double binding. That is, since the alignment member used as the reference position is predetermined in the sheet post-processing apparatus of JP H7-172672A, the “bottom” side of a booklet cannot become the reference position when some finishes are selected.